Light-emitting device

ABSTRACT

It is an object to provide a light-emitting device including a thin film transistor with high electric characteristics and high reliability, and a method for manufacturing the light-emitting device with high productivity. As for a light-emitting device including an inverted staggered thin film transistor of a channel stop type, the inverted staggered thin film transistor includes a gate electrode, a gate insulating film over the gate electrode, a microcrystalline semiconductor film including a channel formation region over the gate insulating film, a buffer layer over the microcrystalline semiconductor film, a channel protective layer which is provided over the buffer layer so as to overlap with the channel formation region of the microcrystalline semiconductor film, a source region and a drain region over the channel protective layer and the buffer layer, and a source electrode and a drain electrode over the source region and the drain region.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light-emitting device including athin film transistor at least in a pixel portion.

2. Description of the Related Art

In recent years, technology that is used to form a thin film transistorusing a semiconductor thin film (with a thickness of from severalnanometers to several hundreds of nanometers, approximately) formed overa substrate that has an insulating surface has been attractingattention. Thin film transistors are applied to a wide range ofelectronic devices like ICs and electro-optical devices, and promptdevelopment of thin film transistors that are to be used as switchingelements in image display devices, in particular, is being pushed.

For switching elements in image display devices, a thin film transistorusing an amorphous semiconductor film, a thin film transistor using apolycrystalline semiconductor film, and the like are used. As a methodfor forming a polycrystalline semiconductor film, technology is known inwhich a pulsed excimer laser beam is processed into a linear shape by anoptical system and used to scan and irradiate an amorphous semiconductorfilm for crystallizing the amorphous semiconductor film.

Also, as switching elements in image display devices, a thin filmtransistor using a microcrystalline semiconductor film is used(Reference 1: Japanese Published Patent Application No. H4-242724 andReference 2: Japanese Published Patent Application No. 2005-49832).

A known conventional method for manufacturing the thin film transistoris that an amorphous silicon film is formed over a gate insulating film;a metal film is formed thereover; and the metal film is irradiated witha diode laser beam to modify the amorphous silicon film into amicrocrystalline silicon film (Reference 3: Toshiaki Arai et al., SID 07DIGEST, 2007, pp. 1370-1373). According to this method, the metal filmformed over the amorphous silicon film is provided to convert opticalenergy of the diode laser beam into thermal energy and should be removedin a later step to complete a thin film transistor. That is, the methodis that an amorphous semiconductor film is heated only by conductionheating from a metal film to form a microcrystalline semiconductor film.

SUMMARY OF THE INVENTION

A thin film transistor using a polycrystalline semiconductor film hasadvantages in that its mobility is two or more orders of magnitudegreater than that of a thin film transistor using an amorphoussemiconductor film and a pixel portion of a display device andperipheral driver circuits thereof can be formed over the samesubstrate. However, the process is more complex because ofcrystallization of a semiconductor film, compared to the case of usingan amorphous semiconductor film; accordingly, there are problems in thatthe yield is decreased and the cost is increased.

In view of the above-mentioned problems, it is an object of the presentinvention to propose a light-emitting device including a thin filmtransistor with high electric characteristics and high reliability.

As for a light-emitting device having an inverted staggered thin filmtransistor of a channel stop type in which a microcrystallinesemiconductor film is used as a channel formation region, the invertedstaggered thin film transistor is formed as follows: a gate insulatingfilm is formed over a gate electrode; a microcrystalline semiconductorfilm (also referred to as a semi-amorphous semiconductor film) whichfunctions as a channel formation region is formed over the gateinsulating film; a buffer layer is formed over the microcrystallinesemiconductor film; a channel protective layer is formed over the bufferlayer so as to overlap with the channel formation region of themicrocrystalline semiconductor film; a pair of source and drain regionsare formed over the channel protective layer and the buffer layer; and apair of source and drain electrodes are formed in contact with thesource and drain regions.

The channel protective layer (also referred to as simply a protectivelayer) is provided over the channel formation region of themicrocrystalline semiconductor film with the buffer layer interposedtherebetween. Thus, damage which is caused in the manufacturing processto the buffer layer over the channel formation region of themicrocrystalline semiconductor film (such as reduction in film thicknessdue to plasma or an etching agent in etching, or oxidation) can beprevented. Therefore, reliability of the thin film transistor can beimproved. Further, since the buffer layer over the channel formationregion of the microcrystalline semiconductor film is not etched, thebuffer layer is not needed to be formed thickly and film-formation timecan be shortened. Note that the channel protective layer functions as anetching stopper in etching for forming the source region and the drainregion and can also be referred to as a channel stopper layer.

For the buffer layer, an amorphous semiconductor film can be used.Preferably, an amorphous semiconductor film containing at least one ofnitrogen, hydrogen, and halogen is used. When the amorphoussemiconductor film contains any one of nitrogen, hydrogen, and halogen,oxidation of crystals included in the microcrystalline semiconductorfilm can be reduced. While the microcrystalline semiconductor film hasan energy gap of 1.1 eV to 1.5 eV, the buffer layer has an energy gap aslarge as 1.6 eV to 1.8 eV and low mobility. The typical mobility of thebuffer layer is a fifth to a tenth of that of the microcrystallinesemiconductor film. Thus, the channel formation region is formed with amicrocrystalline semiconductor film, and the buffer layer serves ahigh-resistance region. The concentration of each of carbon, nitrogen,and oxygen contained in the microcrystalline semiconductor film is setat less than or equal to 3×10¹⁹ atoms/cm³, preferably, less than orequal to 5×10¹⁸ atoms/cm³. The thickness of the microcrystallinesemiconductor film is preferably from 2 nm to 50 nm, more preferably,from 10 nm to 30 nm.

The buffer layer can be formed by a plasma CVD method, a sputteringmethod, or the like. After formation of an amorphous semiconductor film,the surface of the amorphous semiconductor film can be nitrided,hydrogenated, or halogenated through processing of the surface of theamorphous semiconductor film with nitrogen plasma, hydrogen plasma, orhalogen plasma.

By provision of the buffer layer over the surface of themicrocrystalline semiconductor film, oxidation of crystal grainscontained in the microcrystalline semiconductor film can be reduced.Accordingly, the degree of degradation of electric characteristics ofthe thin film transistor can be lowered.

A microcrystalline semiconductor film can be formed over a substratedirectly as a microcrystalline semiconductor film, which is a differentpoint from the case of a polycrystalline semiconductor film.Specifically, a microcrystalline semiconductor film can be formed usingsilicon hydride as a source gas by use of a microwave plasma CVDapparatus with a frequency of greater than or equal to 1 GHz. Amicrocrystalline semiconductor film formed by the above method alsoincludes a microcrystalline semiconductor film which has crystal grainswith a diameter of 0.5 nm to 20 nm in an amorphous semiconductor.Therefore, a crystallization process after formation of thesemiconductor film is not necessary, which is different from the case ofthe polycrystalline semiconductor film; thus, the number of steps inmanufacturing a thin film transistor can be reduced, the yield of thelight-emitting device can be improved, and the cost can be suppressed.In addition, since plasma generated by using microwaves with a frequencyof greater than or equal to 1 GHz has high electron density, siliconhydride which is a source gas can be easily dissociated. Accordingly,compared to the case of using a high-frequency plasma CVD method with afrequency of several tens of MHz to several hundreds of MHz, by use of amicrowave plasma CVD apparatus with a frequency of greater than or equalto 1 GHz, the microcrystalline semiconductor film can be easily formed,a film-formation rate can be increased, and mass productivity of thelight-emitting device can be improved.

In addition, a thin film transistor (TFT) is manufactured using themicrocrystalline semiconductor film, and a light-emitting device ismanufactured using the thin film transistor for a pixel portion, andfurther, for a driver circuit. The thin film transistor using amicrocrystalline semiconductor film has a mobility of 1 cm²/V·sec to 20cm²/V·sec, which is 2 to 20 times higher than that of the thin filmtransistor using an amorphous semiconductor film. Therefore, part of thedriver circuit or the entire driver circuit can be formed over the samesubstrate as that of the pixel portion, so that a system-on-panel can bemanufactured.

The gate insulating film, the microcrystalline semiconductor film, thebuffer layer, the channel protective layer, and the semiconductor filmto which an impurity element imparting one conductivity type is addedwhich forms the source and drain regions may be formed in one reactionchamber, or different reaction chambers according to a kind of a film.

Before a substrate is carried into a reaction chamber to perform filmformation, it is preferable to perform cleaning, flush (washing)treatment (hydrogen flush using hydrogen as a flush substance, silaneflush using silane as a flush substance, or the like), and coating bywhich the inner wall of each reaction chamber is coated with aprotective film (the coating is also referred to as pre-coatingtreatment). Pre-coating treatment is treatment in which plasma treatmentis performed by flowing of a deposition gas in a reaction chamber tocoat the inner wall of the reaction chamber with a thin protective filmwhich is a film to be formed, in advance. By the flush treatment and thepre-coating treatment, a film to be formed can be prevented from beingcontaminated by an impurity element such as oxygen, nitrogen, orfluorine in the reaction chamber.

According to one aspect of the present invention, a light-emittingdevice includes a gate electrode; a gate insulating film over the gateelectrode; a microcrystalline semiconductor film including a channelformation region over the gate insulating film; a buffer layer over themicrocrystalline semiconductor film; a channel protective layer which isprovided over the buffer layer so as to overlap with the channelformation region of the microcrystalline semiconductor film; a sourceregion and a drain region over the channel protective layer and thebuffer layer; and a source electrode and a drain electrode over thesource region and the drain region.

According to another aspect of the present invention, a light-emittingdevice includes a gate electrode; a gate insulating film over the gateelectrode; a microcrystalline semiconductor film including a channelformation region over the gate insulating film; a buffer layer over themicrocrystalline semiconductor film; a channel protective layer which isprovided over the buffer layer so as to overlap with the channelformation region of the microcrystalline semiconductor film; a sourceregion and a drain region over the channel protective layer and thebuffer layer; a source electrode and a drain electrode over the sourceregion and the drain region; and an insulating film which covers part ofthe channel protective layer, the source electrode, and the drainelectrode.

In the above structures, a pixel electrode is provided to beelectrically connected to the source electrode or the drain electrode ofthe channel stop type thin film transistor, and a light-emitting elementand the thin film transistor are electrically connected to each otherthrough the pixel electrode.

A light-emitting device includes a light-emitting element. Examples of alight-emitting element include, in its category, an element whoseluminance is controlled with current or voltage, specifically, aninorganic EL (electroluminescence) element, an organic EL element, andthe like. Further, a display medium whose contrast is changed by anelectric effect, such as an electronic ink, can be used.

In addition, the light-emitting device includes a panel in which alight-emitting element is sealed, and a module in which an IC and thelike including a controller are mounted on the panel. The presentinvention further relates to one mode of an element substrate before thelight-emitting element is completed in a manufacturing process of thelight-emitting device, and the element substrate is provided with meansto supply current to the light-emitting element in each of a pluralityof pixels. Specifically, the element substrate may be in a stateprovided with only a pixel electrode of the light-emitting element, astate after a conductive film to be a pixel electrode is formed andbefore the conductive film is etched to form the pixel electrode, orother states.

Note that a light-emitting device in this specification means an imagedisplay device, a display device, or a light source (including alighting device). Further, the light-emitting device includes any of thefollowing modules in its category: a module to which a connector such asan FPC (flexible printed circuit), TAB (tape automated bonding) tape, ora TCP (tape carrier package) is attached; a module having TAB tape or aTCP which is provided with a printed wiring board at the end thereof;and a module having an IC (integrated circuit) directly mounted on asubstrate provided with a display element by a COG (chip on glass)method.

According to the present invention, a light-emitting device including athin film transistor with high electric characteristics and highreliability can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory view of a light-emitting device of the presentinvention.

FIGS. 2A to 2D are explanatory views of a method for manufacturing alight-emitting device of the present invention.

FIGS. 3A to 3C are explanatory views of a method for manufacturing alight-emitting device of the present invention.

FIGS. 4A to 4D are explanatory views of a method for manufacturing alight-emitting device of the present invention.

FIG. 5 is an explanatory view of a light-emitting device of the presentinvention.

FIGS. 6A to 6D are explanatory views of a method for manufacturing alight-emitting device of the present invention.

FIGS. 7A to 7D show electronic devices to which the present invention isapplied.

FIG. 8 is a block diagram showing a main structure of an electronicdevice to which the present invention is applied.

FIGS. 9A to 9C show a light-emitting device of the present invention.

FIGS. 10A and 10B show a light-emitting device of the present invention.

FIGS. 11A to 11C show a method for manufacturing a light-emitting deviceof the present invention.

FIGS. 12A and 12B show a light-emitting device of the present invention.

FIGS. 13A and 13B are plane views showing a plasma CVD apparatus of thepresent invention.

FIG. 14 is an explanatory view of a light-emitting device of the presentinvention.

FIG. 15 is an explanatory view of a light-emitting device of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiment modes of the present invention will be described in detailwith reference to the accompanying drawings. Note that the presentinvention is not limited to the following description, and it is easilyunderstood by those skilled in the art that modes and details thereofcan be modified in various ways without departing from the spirit andthe scope of the present invention. Therefore, the present inventionshould not be interpreted as being limited to the description of theembodiment modes to be given below. In the structure of the presentinvention to be described below, the same reference numerals arecommonly given to the same components or components having similarfunctions in different drawings, and repetitive description will beomitted.

Embodiment Mode 1

This embodiment mode will describe a thin film transistor which is usedfor a light-emitting device and a manufacturing process of the thin filmtransistor with reference to FIG. 1, FIGS. 2A to 2D, FIGS. 3A to 3C, andFIGS. 4A to 4D. FIG. 1, FIGS. 2A to 2D, and FIGS. 3A to 3C arecross-sectional views showing a thin film transistor and a manufacturingprocess thereof, and FIGS. 4A to 4D are plane views showing a region ina pixel where the thin film transistor and a pixel electrode areconnected to each other. FIG. 1, FIGS. 2A to 2D, and FIGS. 3A to 3C arecross-sectional views showing the thin film transistor in a crosssection taken along a line A-B in FIGS. 4A to 4D, and a manufacturingprocess thereof.

As for a thin film transistor including a microcrystalline semiconductorfilm, an n-type thin film transistor has higher mobility than a p-typethin film transistor; thus, an n-type thin film transistor is moresuitable for a driver circuit. However, in the present invention, eitheran n-type or p-type thin film transistor can be used. With any polarityof a thin film transistor, it is preferable that all the thin filmtransistors formed over one substrate have the same polarity so that thenumber of manufacturing steps is reduced. Here, an n-channel thin filmtransistor will be described.

FIG. 1 shows a bottom gate thin film transistor 74 of a channel stoptype (also referred to as a channel protective type) of this embodimentmode.

In FIG. 1, the channel stop type thin film transistor 74 is providedover a substrate 50. The channel stop type thin film transistor 74includes a gate electrode 51, gate insulating films 52 a and 52 b, amicrocrystalline semiconductor film 61, a buffer layer 62, a channelprotective layer 80, source and drain regions 72, and source and drainelectrodes 71 a, 71 b, and 71 c. A pixel electrode 77 is provided so asto be in contact with the source or drain electrode 71 c. An insulatingfilm 76 is provided so as to cover the thin film transistor 74 and partof the pixel electrode 77. Note that FIG. 1 corresponds to FIG. 4D.

The channel protective layer 80 is provided over a channel formationregion of the microcrystalline semiconductor film 61 with the bufferlayer 62 interposed therebetween. Thus, damage which is caused in themanufacturing process to the buffer layer 62 over the channel formationregion of the microcrystalline semiconductor film 61 (such as reductionin film thickness due to plasma or an etching agent in etching, oroxidation) can be prevented. Therefore, reliability of the thin filmtransistor 74 can be improved. Further, the buffer layer 62 over thechannel formation region of the microcrystalline semiconductor film 61is not etched, so that the buffer layer 62 is not needed to be formedthickly and film-formation time can be shortened.

End portions of the microcrystalline semiconductor film 61 arepositioned more inwardly than those of the gate electrode 51 with whichthe microcrystalline semiconductor film 61 overlaps with the gateinsulating films 52 a and 52 b interposed therebetween, so that themicrocrystalline semiconductor film 61 is provided so as not to extendbeyond the gate electrode 51. Thus, the microcrystalline semiconductorfilm 61 can be formed in a flat region over the gate electrode 51 andthe gate insulating films 52 a and 52 b, and can be a film which coversthe underlying layers adequately and has uniform characteristics(crystalline structure) throughout the film.

Hereinafter, a manufacturing method will be described in detail. Thegate electrode 51 is formed over the substrate 50 (FIG. 2A and FIG. 4A).FIG. 2A is a cross-sectional view showing a cross section taken along aline A-B in FIG. 4A. As the substrate 50, a plastic substrate havingheat resistance that can withstand a processing temperature of themanufacturing process or the like as well as a non-alkaline glasssubstrate manufactured by a fusion method or a float method such as asubstrate of a barium borosilicate glass, an aluminoborosilicate glass,or an aluminosilicate glass, or a ceramic substrate can be used.Alternatively, a metal substrate such as a stainless steel alloysubstrate, provided with an insulating film over the surface, may alsobe used. As the substrate 50, a substrate having a size of 320 mm×400mm, 370 mm×470 mm, 550 mm×650 mm, 600 mm×720 mm, 680 mm×880 mm, 730mm×920 mm, 1000 mm×1200 mm, 1100 mm×1250 mm, 1150 mm×1300 mm, 1500mm×1800 mm, 1900 mm×2200 mm, 2160 mm×2460 mm, 2400 mm×2800 mm, 2850mm×3050 mm, or the like can be used.

The gate electrode 51 is formed of a metal material such as titanium,molybdenum, chromium, tantalum, tungsten, or aluminum, or an alloymaterial thereof. The gate electrode 51 can be formed as follows: aconductive film is formed over the substrate 50 by a sputtering methodor a vacuum evaporation method, a mask is formed by a photolithographytechnique or an ink-jet method over the conductive film, and theconductive film is etched using the mask. Alternatively, the gateelectrode 51 can be formed by discharging a conductive nanopaste ofsilver, gold, copper, or the like by an ink-jet method and baking it.Note that a nitride film formed of the above metal material may beprovided between the substrate 50 and the gate electrode 51 to improveadherence of the gate electrode 51 to the substrate 50 and to prevent,as a barrier metal, diffusion of impurities to a base film and thesubstrate. The gate electrode 51 may have a layered structure, and astructure can be used in which, from the substrate 50 side, an aluminumfilm and a molybdenum film are stacked, a copper film and a molybdenumfilm are stacked, a copper film and a titanium nitride film are stacked,a copper film and a tantalum nitride film are stacked, or the like. Inthe above layered structure, a molybdenum film or a nitride film such asa titanium nitride film or a tantalum nitride film which is formed inthe upper layer has an effect as a barrier metal.

Since semiconductor films and wirings are formed over the gate electrode51, the gate electrode 51 is preferably processed to have tapered endportions so that the semiconductor films and the wirings thereover arenot disconnected. Further, although not illustrated, wirings connectedto the gate electrode can also be formed at the same time when the gateelectrode is formed.

Next, the gate insulating films 52 a and 52 b, a microcrystallinesemiconductor film 53, and a buffer layer 54 are formed in sequence overthe gate electrode 51 (FIG. 2B).

The microcrystalline semiconductor film 53 may be formed over thesurface of the gate insulating film 52 b which is being (or which hasbeen) affected by hydrogen plasma. By formation of a microcrystallinesemiconductor film over a gate insulating film which has been affectedby hydrogen plasma, crystal growth of microcrystal can be accelerated.In addition, lattice distortion at the interface between the gateinsulating film and the microcrystalline semiconductor film can bedecreased, and interface characteristics of the gate insulating film andthe microcrystalline semiconductor film can be improved. Accordingly,electric characteristics and reliability of the microcrystallinesemiconductor film obtained can be improved.

Note that the gate insulating films 52 a and 52 b, the microcrystallinesemiconductor film 53, and the buffer layer 54 may be formedsuccessively without being exposed to the atmosphere. When the gateinsulating films 52 a and 52 b, the microcrystalline semiconductor film53, and the buffer layer 54 are formed successively without beingexposed to the atmosphere, an interface between the films can be formedwithout being contaminated with atmospheric components or impurityelements contained in the atmosphere. Thus, variation in characteristicsof the thin film transistors can be reduced.

The gate insulating films 52 a and 52 b can each be formed using asilicon oxide film, a silicon nitride film, a silicon oxynitride film,or a silicon nitride oxide film by a CVD method, a sputtering method, orthe like. In addition, the gate insulating films 52 a and 52 b can beformed by stacking a silicon nitride film or a silicon nitride oxidefilm, and a silicon oxide film or a silicon oxynitride film in sequence.Further, the gate insulating film can be formed with a three-layerstructure in which a silicon nitride film or a silicon nitride oxidefilm, a silicon oxide film or a silicon oxynitride film, and a siliconnitride film or a silicon nitride oxide film are stacked in sequencefrom the substrate side instead of a two-layer structure. In addition,the gate insulating film may be formed with a single layer of a siliconoxide film, a silicon nitride film, a silicon oxynitride film, or asilicon nitride oxide film. Furthermore, it is preferable to form thegate insulating film by use of a microwave plasma CVD apparatus with afrequency of greater than or equal to 1 GHz. A silicon oxynitride filmor a silicon nitride oxide film formed by use of a microwave plasma CVDapparatus has high resistance to voltage, so that reliability of thethin film transistor formed later can be improved.

As an example of the three-layer structure of the gate insulating film,over the gate electrode, a silicon nitride film or a silicon nitrideoxide film may be formed as a first layer, a silicon oxynitride film maybe formed as a second layer, and a silicon nitride film may be formed asa third layer, and the microcrystalline semiconductor film may be formedover the silicon nitride film that is a top layer. In this case, thesilicon nitride film or the silicon nitride oxide film in the firstlayer is preferably thicker than 50 nm and has an effect as a barrierwhich blocks impurities such as sodium, an effect of preventing ahillock of the gate electrode, an effect of preventing oxidation of thegate electrode, and the like. The silicon nitride film in the thirdlayer has an effect of improving adherence of the microcrystallinesemiconductor film and an effect of preventing oxidation in LP treatmentin which the microcrystalline semiconductor film is irradiated with alaser beam.

When a nitride film such as a silicon nitride film which is very thin isformed over the surface of the gate insulating film in this manner,adherence of the microcrystalline semiconductor film can be improved.The nitride film may be formed by a plasma CVD method, or by nitridationtreatment that is treatment with plasma which is generated by microwavesand has high density and low temperature. In addition, the siliconnitride film or the silicon nitride oxide film may also be formed when areaction chamber is subjected to silane flush treatment.

Note that a silicon oxynitride film means a film that contains moreoxygen than nitrogen and includes oxygen, nitrogen, silicon, andhydrogen at concentrations ranging from 55 at. % to 65 at. %, 1 at. % to20 at. %, 25 at. % to 35 at. %, and 0.1 at. % to 10 at. %, respectively.Further, a silicon nitride oxide film means a film that contains morenitrogen than oxygen and includes oxygen, nitrogen, silicon, andhydrogen at concentrations ranging from 15 at. % to 30 at. %, 20 at. %to 35 at. %, 25 at. % to 35 at. %, and 15 at. % to 25 at. %,respectively.

The microcrystalline semiconductor film 53 is a film which contains asemiconductor having an intermediate structure between amorphous andcrystalline structures (including a single crystal and a polycrystal).This semiconductor is a semiconductor which has a third state that isstable in terms of free energy, and is a crystalline semiconductor whichhas short-range order and lattice distortion, and column-like orneedle-like crystals with a grain size, seen from the film surface, of0.5 nm to 20 nm grown in the direction of a normal line with respect tothe surface of the substrate. In addition, a microcrystallinesemiconductor and an amorphous semiconductor are mixed. Microcrystallinesilicon, which is a typical example of a microcrystalline semiconductor,has a Raman spectrum which is shifted to a lower wave number side than521 cm⁻¹ that is a feature of single crystalline silicon. That is, thepeak of a Raman spectrum of microcrystalline silicon is within the rangefrom 480 cm⁻¹ (that is a feature of amorphous silicon) to 521 cm⁻¹ (thatis a feature of single crystalline silicon). In addition,microcrystalline silicon is made to contain hydrogen or halogen of atleast greater than or equal to 1 at. % for termination of danglingbonds. Moreover, microcrystalline silicon is made to contain a rare gaselement such as helium, argon, krypton, or neon to further enhance itslattice distortion, whereby stability is increased and a favorablemicrocrystalline semiconductor film can be obtained. Such amicrocrystalline semiconductor film is disclosed in, for example, U.S.Pat. No. 4,409,134.

The microcrystalline semiconductor film can be formed by ahigh-frequency plasma CVD method with a frequency of several tens of MHzto several hundreds of MHz or by use of a microwave plasma CVD apparatuswith a frequency of greater than or equal to 1 GHz. The microcrystallinesemiconductor film can be typically formed by a dilution of siliconhydride such as SiH₄, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, or SiF₄ withhydrogen. In addition, by a dilution with one or plural kinds of raregas elements selected from helium, argon, krypton, and neon in additionto silicon hydride and hydrogen, the microcrystalline semiconductor filmcan be formed. In that case, the flow rate ratio of hydrogen to siliconhydride is set to be 5:1 to 200:1, preferably, 50:1 to 150:1, morepreferably, 100:1.

The microcrystalline semiconductor film has low n-type conductivity whenan impurity element for controlling valence electrons is not addedthereto intentionally. Therefore, an impurity element imparting p-typeconductivity may be added to the microcrystalline semiconductor filmwhich functions as a channel formation region of a thin film transistorat the same time as or after formation of the microcrystallinesemiconductor film, so that the threshold voltage can be controlled. Atypical example of the impurity element imparting p-type conductivity isboron, and an impurity gas such as B₂H₆ or BF₃ may be added to siliconhydride at 1 ppm to 1000 ppm, preferably 1 ppm to 100 ppm. Theconcentration of boron is preferably set at 1×10¹⁴ atoms/cm³ to 6×10¹⁶atoms/cm³.

In addition, the oxygen concentration of the microcrystallinesemiconductor film is preferably set at less than or equal to 5×10¹⁹atoms/cm³, more preferably, less than or equal to 1×10¹⁹ atoms/cm³ andeach of the nitrogen concentration and the carbon concentration ispreferably set at less than or equal to 1×10¹⁸ atoms/cm³. By decreasesin concentrations of oxygen, nitrogen, and carbon to be mixed into themicrocrystalline semiconductor film, the microcrystalline semiconductorfilm can be prevented from being changed into an n-type.

The microcrystalline semiconductor film 53 is formed with a thickness ofgreater than 0 nm and less than or equal to 50 nm, preferably, greaterthan 0 nm and less than or equal to 20 nm.

The microcrystalline semiconductor film 53 functions as a channelformation region of a thin film transistor to be formed later. When thethickness of the microcrystalline semiconductor film 53 is within therange described above, a thin film transistor to be formed later is tobe a fully depleted type. In addition, because the microcrystallinesemiconductor film contains microcrystals, it has a lower resistancethan an amorphous semiconductor film. Therefore, a thin film transistorusing the microcrystalline semiconductor film has current-voltagecharacteristics represented by a curve with a steep slope in a risingportion, has an excellent response as a switching element, and can beoperated at high speed. With the use of the microcrystallinesemiconductor film for a channel formation region of a thin filmtransistor, fluctuation of a threshold voltage of a thin film transistorcan be suppressed. Therefore, a light-emitting device with lessvariation of electric characteristics can be manufactured.

The microcrystalline semiconductor film has higher mobility than anamorphous semiconductor film. Thus, with the use of a thin filmtransistor, a channel formation region of which is formed of themicrocrystalline semiconductor film, for switching of a display element,the area of the channel formation region, that is, the area of the thinfilm transistor can be decreased. Accordingly, the area occupied by thethin film transistor in a single pixel is decreased, and an apertureratio of the pixel can be increased. As a result of this, alight-emitting device with high resolution can be manufactured.

In addition, the microcrystalline semiconductor film has needle-likecrystals which have grown longitudinally from the lower side. Themicrocrystalline semiconductor film has a mixed structure of amorphousand crystalline structures, and it is likely that a crack is generatedand a gap is formed between the crystalline region and the amorphousregion due to local stress. A new radical may be interposed into thisgap and cause crystal growth. Because the upper crystal face is larger,a crystal is likely to grow upward into a needle shape. Even if themicrocrystalline semiconductor film grows longitudinally as describedabove, the growth rate is a tenth to a hundredth of the film-formationrate of an amorphous semiconductor film.

The buffer layer 54 can be formed by a plasma CVD method using a silicongas (a silicon hydride gas or a silicon halide gas) such as SiH₄, Si₂H₆,SiH₂Cl₂, SiHCl₃, SiCl₄, or SiF₄. Alternatively, by a dilution of silanementioned above with one or plural kinds of rare gas elements selectedfrom helium, argon, krypton, and neon, an amorphous semiconductor filmcan be formed. With the use of hydrogen at a flow rate which is 1 to 20times, preferably, 1 to 10 times, more preferably, 1 to 5 times higherthan that of silicon hydride, a hydrogen-containing amorphoussemiconductor film can be formed. With the use of silicon hydridementioned above and nitrogen or ammonia, a nitrogen-containing amorphoussemiconductor film can be formed. With the use of silicon hydridementioned above and a gas containing fluorine, chlorine, bromine, oriodine (F₂, Cl₂, Br₂, I₂, HF, HCl, HBr, HI, or the like), an amorphoussemiconductor film containing fluorine, chlorine, bromine, or iodine canbe formed.

Alternatively, as the buffer layer 54, an amorphous semiconductor filmcan be formed by sputtering with hydrogen or a rare gas using anamorphous semiconductor as a target. In this case, by inclusion ofammonia, nitrogen, or N₂O in an atmosphere, a nitrogen-containingamorphous semiconductor film can be formed. Alternatively, by inclusionof a gas containing fluorine, chlorine, bromine, or iodine (F₂, Cl₉,Br₂, I₂, HF, HCl, HBr, HI, or the like) in an atmosphere, an amorphoussemiconductor film containing fluorine, chlorine, bromine, or iodine canbe formed.

Still alternatively, the buffer layer 54 may be formed by formation ofan amorphous semiconductor film over the surface of the microcrystallinesemiconductor film 53 by a plasma CVD method or a sputtering method andthen by hydrogenation, nitridation, or halogenation of the surface ofthe amorphous semiconductor film through processing of the surface ofthe amorphous semiconductor film with hydrogen plasma, nitrogen plasma,halogen plasma, or plasma of a rare gas (helium, argon, krypton, orneon).

The buffer layer 54 is preferably formed using an amorphoussemiconductor film. Therefore, when the buffer layer 54 is formed by ahigh-frequency plasma CVD method with a frequency of several tens of MHzto several hundreds of MHz or a microwave plasma CVD method, formationconditions are preferably controlled so that an amorphous semiconductorfilm can be obtained.

The buffer layer 54 is preferably formed with a thickness of 10 nm to 50nm, inclusive. The total concentration of nitrogen, carbon, and oxygencontained in the buffer layer is preferably set at 1×10²⁰ atoms/cm³ to15×10²⁰ atoms/cm³. With this concentration, also the buffer layer 54having a thickness of 10 nm to 50 nm, inclusive can function as ahigh-resistance region.

Alternatively, the buffer layer 54 may be formed with a thickness of 150nm to 200 nm, inclusive, and the concentration of each of carbon,nitrogen, and oxygen contained in the buffer layer 54 may be set at lessthan or equal to 3×10¹⁹ atoms/cm³, preferably, less than or equal to5×10¹⁸ atoms/cm³.

By formation of an amorphous semiconductor film or an amorphoussemiconductor film containing hydrogen, nitrogen, or halogen over thesurface of the microcrystalline semiconductor film 53 as a buffer layer,the surfaces of crystal grains contained in the microcrystallinesemiconductor film 53 can be prevented from being naturally oxidized.That is, by formation of the buffer layer over the surface of themicrocrystalline semiconductor film 53, the microcrystal grains can beprevented from being oxidized. Since the buffer layer includes hydrogenand/or fluorine, oxygen can be prevented from entering themicrocrystalline semiconductor film.

The buffer layer 54 is formed using an amorphous semiconductor film oran amorphous semiconductor film containing hydrogen, nitrogen, orhalogen, so that the buffer layer 54 has higher resistance than themicrocrystalline semiconductor film which functions as a channelformation region. Therefore, in a thin film transistor to be formedlater, the buffer layer formed between source and drain regions and themicrocrystalline semiconductor film functions as a high-resistanceregion. Accordingly, the off current of the thin film transistor can bereduced. When the thin film transistor is used as a switching element ofa light-emitting device, the contrast of the light-emitting device canbe improved.

Next, the channel protective layer 80 is formed over the buffer layer 54so as to overlap with the channel formation region of themicrocrystalline semiconductor film 53 (FIG. 2C). The channel protectivelayer 80 may also be formed successively after the gate insulating films52 a and 52 b, the microcrystalline semiconductor film 53, and thebuffer layer 54 are formed, without being exposed to the atmosphere.When the thin films that are stacked are formed successively withoutexposing the substrate to the atmosphere, the productivity can beimproved.

The channel protective layer 80 can be formed using an inorganicmaterial (such as silicon oxide, silicon nitride, silicon oxynitride, orsilicon nitride oxide). A photosensitive or non-photosensitive organicmaterial (organic resin material, e.g., polyimide, acrylic, polyamide,polyimideamide, resist, or benzocyclobutene), a film made of pluralkinds of these materials, or a stacked film of them may also be used.Alternatively, siloxane may be used. As a manufacturing method of thechannel protective layer 80, a vapor deposition method such as a plasmaCVD method or a thermal CVD method, or a sputtering method can be used.A coating method such as a spin coating method or a droplet dischargingmethod which is a wet method, a printing method (such as screen printingor offset printing by which a pattern is formed), or the like can alsobe used. The channel protective layer 80 may be formed and thenpatterned by etching, or may be formed as selected by a dropletdischarging method.

Next, the microcrystalline semiconductor film 53 and the buffer layer 54are patterned by etching, and a stack of the microcrystallinesemiconductor film 61 and the buffer layer 62 is formed (FIG. 2D). Themicrocrystalline semiconductor film 61 and the buffer layer 62 can beformed by forming a mask by a photolithography technique or a dropletdischarging method and etching the microcrystalline semiconductor film53 and the buffer layer 54 using the mask. FIG. 2D is a cross-sectionalview of a cross section taken along a line A-B in FIG. 4B.

The end portions of the microcrystalline semiconductor film 61 and thebuffer layer 62 can be etched to have a tapered shape. The taper angleof the end portions is 30° to 90°, preferably 45° to 80°. Thus,disconnection of a wiring due to a step shape can be prevented.

Next, a semiconductor film 63 to which an impurity element imparting oneconductivity type is added (hereinafter, the semiconductor film 63) andconductive films 65 a to 65 c are formed over the gate insulating film52 b, the microcrystalline semiconductor film 61, the buffer layer 62,and the channel protective layer 80 (FIG. 3A). A mask 66 is formed overthe semiconductor film 63 and the conductive films 65 a to 65 c. Themask 66 is formed by a photolithography technique or an ink-jet method.

In the case where an n-channel thin film transistor is formed using thesemiconductor film 63, phosphorus may be added as a typical impurityelement to the semiconductor film 63, and an impurity gas such as PH₃may be added to silicon hydride. In addition, when a p-channel thin filmtransistor is formed, boron may be added as a typical impurity element,and an impurity gas such as B₂H₆ may be added to silicon hydride. Thesemiconductor film 63 can be formed using a microcrystallinesemiconductor film or an amorphous semiconductor film and may have athickness of from 2 nm to 50 nm (preferably, from 10 nm to 30 nm).

It is preferable that the conductive film be formed using a single layeror a stacked layer of aluminum, copper, or an aluminum alloy to which anelement to improve resistance to heat or an element which prevents ahillock such as silicon, titanium, neodymium, scandium, or molybdenum isadded. Alternatively, the conductive film may have a layered structurein which a film on the side in contact with the semiconductor film towhich an impurity imparting one conductivity type is added is formed oftitanium, tantalum, molybdenum, tungsten, or a nitride of any of theseelements and an aluminum film or an aluminum alloy film is formedthereover. Still alternatively, the conductive film may have a layeredstructure in which an aluminum film or an aluminum alloy film issandwiched between upper and lower films of titanium, tantalum,molybdenum, tungsten, or a nitride of any of these elements. Here, asthe conductive film, a conductive film with a three-layer structure inwhich the conductive films 65 a to 65 c are stacked is described. Alayered conductive film in which molybdenum films are used as theconductive films 65 a and 65 c and an aluminum film is used as theconductive film 65 b, or a layered conductive film in which titaniumfilms are used as the conductive films 65 a and 65 c and an aluminumfilm is used as the conductive film 65 b can be given.

The conductive films 65 a to 65 c are formed by a sputtering method or avacuum evaporation method. Alternatively, the conductive films 65 a to65 c may be formed by discharging a conductive nanopaste of silver,gold, copper, or the like by a screen printing method, an ink-jetmethod, or the like and baking it.

Next, the conductive films 65 a to 65 c are etched using the mask 66 toform source and drain electrodes 71 a to 71 c (FIG. 3B). When theconductive films 65 a to 65 c are subjected to wet etching as in thisembodiment mode as shown in FIG. 3B, the conductive films 65 a to 65 care isotropically etched. Thus, end portions of the mask 66 and endportions of the source and drain electrodes 71 a to 71 c are notaligned, and the end portions of the source and drain electrodes 71 a to71 c further recede. After that, the semiconductor film 63 is etchedusing the mask 66 to form source and drain regions 72 (FIG. 3C). Notethat the buffer layer 62 is not etched because the channel protectivelayer 80 functions as a channel stopper.

The end portions of the source and drain electrodes 71 a to 71 c are notaligned with the end portions of the source and drain regions 72, andthe end portions of the source and drain regions 72 are formed outsideof the end portions of the source and drain electrodes 71 a to 71 c.After that, the mask 66 is removed. Note that FIG. 3C is across-sectional view of a cross section taken along a line A-B in FIG.4C. As shown in FIG. 4C, it can be seen that the end portions of thesource and drain regions 72 are positioned outside of the end portionsof the source and drain electrodes 71 a to 71 c. In other words, it canbe seen that an area of the source and drain regions 72 is larger thanthat of the source and drain electrodes 71 a to 71 c. One of the sourceand drain electrodes also functions as a source or drain wiring.

With such a shape as shown in FIG. 3C in which the end portions of thesource and drain electrodes 71 a to 71 c are not aligned with the endportions of the source and drain regions 72, the end portions of thesource and drain electrodes 71 a to 71 c are more apart from each other;therefore, leakage current and short circuit between the source anddrain electrodes can be prevented. In other words, it can be seen thatthe source and drain regions 72 extend beyond the end portions of thesource and drain electrodes 71 a to 71 c, and a distance between the endportions of the source and drain regions 72 facing each other is shorterthan a distance between the end portions of the source and drainelectrodes 71 a to 71 c facing each other. Accordingly, a thin filmtransistor with high reliability and high resistance to voltage can bemanufactured.

Through the above-described process, the channel stop (protective) typethin film transistor 74 can be formed.

The buffer layer 62 below the source and drain regions 72 and the bufferlayer 62 over the channel formation region of the microcrystallinesemiconductor film 61 are a continuous film formed using the samematerial at the same time. The buffer layer 62 over the microcrystallinesemiconductor film 61 blocks external air and an etching residue withhydrogen included therein and protects the microcrystallinesemiconductor film 61.

The buffer layer 62 which does not include an impurity element impartingone conductivity type is provided, whereby an impurity element impartingone conductivity type, which is included in the source and drainregions, and an impurity element imparting one conductivity type, whichis used for controlling threshold voltage of the microcrystallinesemiconductor film 61, can be prevented from being mixed to each other.When impurity elements imparting one conductivity type are mixed witheach other, a recombination center is generated, which leads to flow ofleakage current and loss of the effect of reducing off current.

By provision of the buffer layer and the channel protective layer asdescribed above, a channel stop type thin film transistor with highresistance to voltage, in which leakage current is reduced, can bemanufactured. Accordingly, the thin film transistor has high reliabilityand can be suitably used for a light-emitting device to which a voltageof 5 V is applied.

Next, the pixel electrode 77 is formed so as to be in contact with thesource or drain electrode 71 a to 71 c. The insulating film 76 is formedover the source and drain electrodes 71 a to 71 c, the source and drainregions 72, the channel protective layer 80, the gate insulating film 52b, and the pixel electrode 77. The insulating film 76 can be formed in amanner similar to the gate insulating films 52 a and 52 b. Note that theinsulating film 76 prevents intrusion of a contaminating impurity suchas an organic matter, a metal, or water vapor contained in theatmosphere; thus, a dense film is preferably used for the insulatingfilm 76.

The buffer layer 62 is preferably formed with a thickness of 10 nm to 50nm, inclusive. In addition, the total concentration of nitrogen, carbon,and oxygen contained in the buffer layer is preferably set at 1×10²⁰atoms/cm³ to 15×10²⁰ atoms/cm³. With the above concentration, also thebuffer layer 62 having a thickness of 10 nm to 50 nm, inclusive, canfunction as a high-resistance region.

Alternatively, the buffer layer 62 may be formed with a thickness of 150nm to 200 nm, inclusive, and the concentration of carbon, nitrogen, andoxygen contained in the buffer layer 62 may be set at less than or equalto 3×10¹⁹ atoms/cm³, preferably, less than or equal to 5×10¹⁸ atoms/cm³.In this case, when the insulating film 76 is formed of a silicon nitridefilm, the oxygen concentration in the buffer layer 62 can be set at lessthan or equal to 5×10¹⁹ atoms/cm³, preferably, less than or equal to1×10¹⁹ atoms/cm³.

Next, the insulating film 76 is etched so that part of the pixelelectrode 77 is exposed. A light-emitting element is formed to be incontact with an exposed region of the pixel electrode 77, so that thethin film transistor 74 and the light-emitting element can beelectrically connected to each other. For example, a light-emittinglayer may be formed over the pixel electrode 77, and a counter electrodemay be formed over the light-emitting layer.

For the pixel electrode 77, a conductive material having alight-transmitting property, such as indium oxide which containstungsten oxide, indium zinc oxide which contains tungsten oxide, indiumoxide which contains titanium oxide, indium tin oxide which containstitanium oxide, indium tin oxide (hereinafter ITO), indium zinc oxide,or indium tin oxide to which silicon oxide has been added can be used.

The pixel electrode 77 can be formed using a conductive compositioncontaining a conductive high-molecular compound (also referred to as aconductive polymer). It is preferable that the pixel electrode formedusing the conductive composition have a sheet resistance of less than orequal to 10000 Ω/square and a light transmittance of greater than orequal to 70% at a wavelength of 550 nm. In addition, it is preferablethat the resistivity of the conductive high-molecular compound containedin the conductive composition be less than or equal to 0.1Ω·cm.

As a conductive high-molecular compound, a so-called π electronconjugated conductive high-molecular compound can be used. For example,polyaniline or a derivative thereof, polypyrrole or a derivativethereof, polythiophene or a derivative thereof, and a copolymer of twoor more kinds of them can be given.

The end portions of the source and drain regions and the end portions ofthe source and drain electrodes may be aligned with each other. FIG. 14shows a thin film transistor 79 of a channel stop type in which the endportions of the source and drain regions and the end portions of thesource and drain electrodes are aligned with each other. When the sourceand drain electrodes and the source and drain regions are subjected todry etching, a shape like the thin film transistor 79 can be obtained.Alternatively, also when the semiconductor film to which an impurityelement imparting one conductivity type is added is etched using thesource and drain electrodes as a mask to form the source and drainregions, a shape like the thin film transistor 79 can be obtained.

When the thin film transistor is formed as a channel stop type thin filmtransistor, reliability of the thin film transistor can be improved. Byformation of a channel formation region with a microcrystallinesemiconductor film, a field-effect mobility of 1 cm²/V·sec to 20cm²/V·sec can be achieved. Accordingly, this thin film transistor can beused as a switching element of a pixel in a pixel portion and as anelement included in a scanning line (gate line) driver circuit.

According to this embodiment mode, a light-emitting device including athin film transistor with high electric characteristics and highreliability can be manufactured.

Embodiment Mode 2

This embodiment mode will describe an example of a thin film transistorwhose shape is different from that of Embodiment Mode 1. Except theshape, the thin film transistor can be formed in a similar manner toEmbodiment Mode 1; thus, repetitive description of the same componentsor components having similar functions as in Embodiment Mode 1 andmanufacturing steps for forming those components will be omitted.

This embodiment mode will describe a thin film transistor which is usedfor a light-emitting device and a manufacturing process of the thin filmtransistor with reference to FIG. 5, FIGS. 6A to 6D, and FIG. 15. FIG. 5and FIG. 15 are cross-sectional views showing a thin film transistor anda pixel electrode, and FIGS. 6A to 6D are plane views showing a regionin a pixel where the thin film transistor and the pixel electrode areconnected to each other. FIG. 5 and FIG. 15 are cross-sectional viewsshowing the thin film transistor in a cross section taken along a lineQ-R in FIGS. 6A to 6D, and a manufacturing process thereof.

FIG. 5 and FIGS. 6A to 6D show a bottom gate thin film transistor 274 ofa channel stop type (also referred to as a channel protective type) ofthis embodiment mode.

In FIG. 5, the channel stop type thin film transistor 274 is providedover a substrate 250. The channel stop thin film transistor 274 includesa gate electrode 251, gate insulating films 252 a and 252 b, amicrocrystalline semiconductor film 261, a buffer layer 262, a channelprotective layer 280, source and drain regions 272, and source and drainelectrodes 271 a, 271 b, and 271 c. An insulating film 276 is providedso as to cover the thin film transistor 274. A pixel electrode 277 isprovided so as to be in contact with the source or drain electrode 271 cin a contact hole formed in the insulating film 276. Note that FIG. 5corresponds to FIG. 6D.

The channel protective layer 280 is provided over a channel formationregion of the microcrystalline semiconductor film 261 with the bufferlayer 262 interposed therebetween. Thus, damage which is caused in themanufacturing process to the buffer layer 262 over the channel formationregion of the microcrystalline semiconductor film 261 (such as reductionin film thickness due to radicals in plasma or an etching agent inetching, or oxidation) can be prevented. Therefore, reliability of thethin film transistor 274 can be improved. The buffer layer 262 over thechannel formation region of the microcrystalline semiconductor film 261is not etched, so that the buffer layer 262 is not needed to be formedthickly and film-formation time can be shortened.

Hereinafter, a manufacturing method will be described with reference toFIGS. 6A to 6D. The gate electrode 251 is formed over the substrate 250(FIG. 6A). The gate insulating films 252 a and 252 b are formed over thegate electrode 251, and the microcrystalline semiconductor film 261 andthe buffer layer 262 are formed thereover. Over the buffer layer 262,the channel protective layer 280 is formed so as to overlap with thechannel formation region of the microcrystalline semiconductor film(FIG. 6B).

Embodiment Mode 1 shows an example in which, after formation of thechannel protective layer 80, the microcrystalline semiconductor film 53and the buffer layer 54 are processed into the island-shapedmicrocrystalline semiconductor film 61 and the island-shaped bufferlayer 62, respectively, by etching. However, this embodiment mode showsan example in which the microcrystalline semiconductor film and thebuffer layer are etched at the same time when a conductive film to bethe source and drain electrodes and a semiconductor film to which animpurity element imparting one conductivity type is added are etched.Therefore, the microcrystalline semiconductor film, the buffer layer,the semiconductor film to which an impurity element imparting oneconductivity type is added, and the conductive film to be the source anddrain electrodes are etched using the same mask. When themicrocrystalline semiconductor film, the buffer layer, the semiconductorfilm to which an impurity element imparting one conductivity type isadded, and the conductive film to be the source and drain electrodes areetched by one etching process, the manufacturing process can besimplified, and the number of masks used in the etching process can bereduced.

The microcrystalline semiconductor film, the buffer layer, thesemiconductor film to which an impurity element imparting oneconductivity type is added, and the conductive film are etched, so thatthe microcrystalline semiconductor film 261, the buffer layer 262, thesource and drain regions 272, and the source and drain electrodes 271 ato 271 c are formed. In this manner, the channel stop type thin filmtransistor 274 is formed (FIG. 6C). The insulating film 276 is formed soas to cover the thin film transistor 274, and the contact hole whichexposes the source or drain electrode 271 c is formed. The pixelelectrode 277 is formed in the contact hole, so that the thin filmtransistor 274 and the pixel electrode 277 are electrically connected toeach other (FIG. 6D).

The end portions of the source and drain regions and the end portions ofthe source and drain electrodes may be aligned with each other. FIG. 15shows a thin film transistor 279 of a channel stop type in which the endportions of the source and drain regions and the end portions of thesource and drain electrodes are aligned with each other. When the sourceand drain electrodes and the source and drain regions are subjected todry etching, a shape like the thin film transistor 279 can be obtained.Alternatively, also when the semiconductor film to which an impurityelement imparting one conductivity type is added is etched using thesource and drain electrodes as a mask to form the source and drainregions, a shape like the thin film transistor 279 can be obtained.

When the thin film transistor is formed as a channel stop type thin filmtransistor, reliability of the thin film transistor can be improved. Byformation of a channel formation region with a microcrystallinesemiconductor film, a field-effect mobility of 1 cm²/V·sec to 20cm²/V·sec can be achieved. Accordingly, this thin film transistor can beused as a switching element of a pixel in a pixel portion and as anelement included in a scanning line (gate line) driver circuit.

According to this embodiment mode, a light-emitting device including athin film transistor with high electric characteristics and highreliability can be manufactured.

Embodiment Mode 3

This embodiment mode will describe an example of a manufacturing processin which a microcrystalline semiconductor film is irradiated with alaser beam.

A gate electrode is formed over a substrate, and a gate insulating filmis formed so as to cover the gate electrode. Then, a microcrystallinesilicon (SAS) film is formed as a microcrystalline semiconductor filmover the gate insulating film. The thickness of the microcrystallinesemiconductor film is greater than or equal to 1 nm and less than 15 nm,preferably 2 nm to 10 nm, inclusive. In particular, the microcrystallinesemiconductor film with a thickness of 5 nm (4 nm to 8 nm) has highabsorptance of a laser beam and improves productivity.

In the case where the microcrystalline semiconductor film is formed overthe gate insulating film by a plasma CVD method or the like, near theinterface between the gate insulating film and a semiconductor filmwhich contains crystals, a region which contains more amorphouscomponents than the semiconductor film which contains crystals (heresuch a region is referred to as an interface region) is formed in somecases. In addition, in the case where an ultra-thin microcrystallinesemiconductor film with a thickness of about less than or equal to 10 nmis formed by a plasma CVD method or the like, although a semiconductorfilm which contains microcrystal grains can be formed, it is difficultto obtain a semiconductor film which contains microcrystal grains whichhas high quality uniformly throughout the film. In these cases, a laserprocess of irradiation with a laser beam to be described below iseffective.

Next, the surface of the microcrystalline silicon film is irradiatedwith a laser beam having such an energy density that themicrocrystalline silicon film is not melted. This laser process(hereinafter also referred to as “LP”) of this embodiment mode involvessolid-phase crystal growth which is performed by radiation heatingwithout the microcrystalline silicon film being melted. That is, theprocess utilizes a critical region where a deposited microcrystallinesilicon film is not brought into a liquid phase, and in that sense, theprocess can also be referred to as “critical growth”.

The laser beam can affect a region to the interface between themicrocrystalline silicon film and the gate insulating film. Accordingly,using the crystals on the surface side of the microcrystalline siliconfilm as nuclei, solid-phase crystal growth advances from the surfacetoward the interface with the gate insulating film, and roughlycolumn-like crystals grow. The solid-phase crystal growth by the LPprocess is not to increase the size of crystal grains but rather toimprove crystallinity in a film thickness direction.

In the LP process, for example, a microcrystalline silicon film over aglass substrate of 730 mm×920 mm can be processed by a single laser beamscan, by collecting a laser beam into a long rectangular shape (a linearlaser beam). In this case, the proportion of overlap of linear laserbeams (the overlap rate) is set to be 0% to 90% (preferably, 0% to 67%).Accordingly, processing time for each substrate can be shortened, andthe productivity can be increased. The shape of the laser beam is notlimited to a linear shape, and similar processing can be conducted usinga planar laser beam In addition, the LP process of this embodiment modeis not limited to be used for the glass substrate of the above size andcan be used for substrates of various sizes.

The LP process has effects in improving crystallinity of an interfaceregion with the gate insulating film and improving electriccharacteristics of a thin film transistor having a bottom gate structurelike the thin film transistor of this embodiment mode.

In such critical growth, there is also a feature in that unevenness (aprojecting body called a ridge), which is observed on the surface ofconventional low-temperature polysilicon, is not formed and thesmoothness of silicon surface is maintained after the LP process.

A crystalline silicon film which is obtained by the action of the laserbeam directly on the microcrystalline silicon film after the formationas in this embodiment mode is distinctly different in growth mechanismand film quality from a conventional microcrystalline silicon film whichis obtained by being just deposited and a microcrystalline silicon filmwhich is modified by conduction heating (the one disclosed in Reference1). In this specification, a crystalline semiconductor film which isobtained through LP process performed to a microcrystallinesemiconductor film after the formation is referred to as an LPSAS film.

After the microcrystalline semiconductor film such as an LPSAS film isformed, an amorphous silicon (a-Si:H) film is formed as a buffer layerby a plasma CVD method at 300° C. to 400° C. By formation of theamorphous silicon film, hydrogen is supplied to the LPSAS film, and thesame effect as in the case of hydrogenation of the LPSAS film can beachieved. In other words, by formation of the amorphous silicon filmover the LPSAS film, hydrogen is diffused into the LPSAS film, so that adangling bond can be terminated.

Subsequent manufacturing steps are similar to those in EmbodimentMode 1. A channel protective layer is formed, and a mask is formedthereover. Next, the microcrystalline semiconductor film and the bufferlayer are etched using the mask. Then, a semiconductor film to which animpurity element imparting one conductivity type is added and aconductive film are formed, and a mask is formed over the conductivefilm. The conductive film is etched using the mask, so that source anddrain electrodes are formed. Further, using the same mask, thesemiconductor film to which an impurity element imparting oneconductivity type is added is etched using the channel protective layeras an etching stopper, so that source and drain regions are formed.

Through the above process, a channel stop type thin film transistor canbe formed, and a light-emitting device including the channel stop typethin film transistor can be manufactured.

This embodiment mode can be freely combined with Embodiment Mode 1 or 2.

Embodiment Mode 4

This embodiment mode will describe an example of a manufacturing processof a light-emitting device in Embodiment Modes 1 to 3 in detail.Therefore, repetitive description of the same components or componentshaving similar functions as in Embodiment Modes 1 to 3 and manufacturingsteps for forming those components will be omitted.

In Embodiment Modes 1 to 3, before the microcrystalline semiconductorfilm is formed, a reaction chamber may be subjected to cleaning andflush (washing) treatment (hydrogen flush using hydrogen as a flushsubstance, silane flush using silane as a flush substance, or the like).By the flush treatment, a film to be formed can be prevented from beingcontaminated by an impurity such as oxygen, nitrogen, or fluorine in areaction chamber.

By the flush treatment, an impurity such as oxygen, nitrogen, orfluorine in a reaction chamber can be removed. For example, silane flushtreatment is performed in the following manner: a plasma CVD apparatusis used, and monosilane is used as a flush substance and introduced to achamber at a gas flow rate of 8 SLM to 10 SLM for 5 to 20 minutes,preferably 10 to 15 minutes. Note that 1 SLM is 1000 sccm, that is, 0.06m³/h.

The cleaning can be performed with the use of, for example, fluorineradicals. Note that a reaction chamber can be cleaned with the use offluorine radicals in the following manner: carbon fluoride, nitrogenfluoride, or fluorine is introduced to a plasma generator providedoutside the reaction chamber and the gas is dissociated, and thefluorine radials are introduced to the reaction chamber.

The flush treatment may also be performed before the gate insulatingfilm, the buffer layer, the channel protective layer, and thesemiconductor film to which an impurity element imparting oneconductivity type is added are formed. Note that the flush treatment iseffective when it is performed after cleaning.

Before a substrate is carried into a reaction chamber to perform filmformation, the inner wall of each reaction chamber may be coated with aprotective film that is a film to be formed (this coating is alsoreferred to as pre-coating treatment). Pre-coating treatment istreatment in which plasma treatment is performed by flowing of adeposition gas in a reaction chamber to coat the inner wall of thereaction chamber with a thin protective film in advance. For example,before a microcrystalline silicon film is formed as the microcrystallinesemiconductor film, pre-coating treatment may be performed in which theinner wall of the reaction chamber is coated with an amorphous siliconfilm with a thickness of 0.2 μm to 0.4 μm. Flush treatment may beperformed after pre-coating treatment (hydrogen flush, silane flush, orthe like). In the case of performing cleaning and pre-coating treatment,it is necessary that a substrate be carried out from a reaction chamber.However, in the case of performing flush treatment (hydrogen flush,silane flush, or the like), a substrate may be in a reaction chamberbecause plasma treatment is not performed.

A protective film formed of an amorphous silicon film is formed on theinner wall of a reaction chamber in which a microcrystalline siliconfilm is formed, and hydrogen plasma treatment is performed before filmformation. In this case, the protective film is etched and an extremelysmall amount of silicon is deposited on a substrate. The silicon can bea nucleus of crystal growth.

By the pre-coating treatment, a film to be formed can be prevented frombeing contaminated by an impurity such as oxygen, nitrogen, or fluorinein a reaction chamber.

The pre-coating treatment may be performed before formation of a gateinsulating film and a semiconductor film to which an impurity elementimparting one conductivity type is added.

An example of a method for forming a gate insulating film, amicrocrystalline semiconductor film, and a buffer layer is described indetail.

FIGS. 13A and 13B each show an example of a plasma CVD apparatus whichcan be used for the present invention. FIGS. 13A and 13B each show amicrowave plasma CVD apparatus which can perform successive filmformation. FIGS. 13A and 13B are plane views each schematically showinga microwave plasma CVD apparatus. A loading chamber 1110, an unloadingchamber 1115, and reaction chambers (1) 1111 to (4) 1114 are providedaround a common chamber 1120. Gate valves 1122 to 1127 are providedbetween the common chamber 1120 and each chamber so that treatment ineach chamber does not have influence on treatment in other chambers.Note that the number of reaction chambers is not limited to four, andthe number of reaction chambers may be more than four or less than four.When the number of reaction chambers is large, reaction chambers can beallocated according to a kind of a film to be formed; thus, the numberof cleaning of the reaction chamber can be reduced. FIG. 13A shows anexample of a microwave plasma CVD apparatus provided with four reactionchambers, and FIG. 13B shows an example of a microwave plasma CVDapparatus provided with three reaction chambers.

An example is described in which a gate insulating layer, amicrocrystalline semiconductor film, a buffer layer, and a channelprotective layer are formed using a plasma CVD apparatus shown in FIGS.13A and 13B. Substrates are set in a cassette 1128 and a cassette 1129of the loading chamber 1110 and the unloading chamber 1115, andtransferred to the reaction chambers (1) 1111 to (4) 1114 by a transferunit 1121 of the common chamber 1120. In this apparatus, reactionchambers can be allocated according to the films to be deposited, and aplurality of different films can be formed successively without beingexposed to the atmosphere. In addition, the reaction chamber is alsoused as a reaction chamber for performing an etching process or laserirradiation process, in addition to a film-formation process. Whenreaction chambers for various processes are provided, various processescan be performed without exposing the substrate to the atmosphere.

In each of the reaction chambers (1) to (4), the gate insulating film,the microcrystalline semiconductor film, the buffer layer, and thechannel protective layer are stacked. In this case, the plurality ofdifferent kinds of films can be stacked successively by changing sourcegases. Further, in this case, after the gate insulating film is formed,silicon hydride such as silane is introduced to the reaction chamber sothat an oxygen residue is reacted with silicon hydride, and the reactantis ejected outside the reaction chamber; thus, the concentration of anoxygen residue in the reaction chamber can be reduced. Accordingly, theconcentration of oxygen contained in the microcrystalline semiconductorfilm can be reduced. In addition, crystal grains included in themicrocrystalline semiconductor film can be prevented from beingoxidized.

Further, in a plasma CVD apparatus, films of one kind may be formed in aplurality of reaction chambers in order to improve productivity. Whenfilms of one kind can be formed in a plurality of reaction chambers,films can be concurrently formed over a plurality of substrates. Forexample, in FIG. 13A, the reaction chambers (1) and (2) are used asreaction chambers in each of which a microcrystalline semiconductor filmis formed, the reaction chamber (3) is used as a reaction chamber inwhich an amorphous semiconductor film is formed, and the reactionchamber (4) is used as a reaction chamber in which a channel protectivelayer is formed. In the case where a plurality of substrates isconcurrently treated as described above, a plurality of reactionchambers is provided, in each of which a film with a low deposition rateis formed, so that productivity can be improved.

Before a substrate is carried into a reaction chamber to perform filmformation, it is preferable to perform cleaning, flush (washing)treatment (hydrogen flush, silane flush, or the like), and coating bywhich the inner wall of each reaction chamber is coated with aprotective film that is a film to be formed (this coating is alsoreferred to as pre-coating treatment). Pre-coating treatment istreatment in which plasma treatment is performed by flowing of adeposition gas in a reaction chamber to coat the inner wall of thereaction chamber with a thin protective film in advance. For example,before a microcrystalline silicon film is formed as the microcrystallinesemiconductor film, pre-coating treatment may be performed in which theinner wall of the reaction chamber is coated with an amorphous siliconfilm with a thickness of 0.2 μm to 0.4 μm. Flush treatment (hydrogenflush, silane flush, or the like) may be performed after pre-coatingtreatment. In the case of performing cleaning and pre-coating treatment,it is necessary that the substrate be carried out from a reactionchamber. However, in the case of performing flush treatment (hydrogenflush, silane flush, or the like), a substrate may be in a reactionchamber because plasma treatment is not performed.

A protective film formed of an amorphous silicon film is formed on theinner wall of a reaction chamber in which a microcrystalline siliconfilm is formed, and hydrogen plasma treatment is performed before filmformation. In this case, the protective film is etched and an extremelysmall amount of silicon is deposited on a substrate. The silicon can bea nucleus of crystal growth.

In this manner, with use of the microwave plasma CVD apparatus in whichthe plurality of chambers is connected, the gate insulating film, themicrocrystalline semiconductor film, the buffer layer, the channelprotective layer, and the semiconductor film to which an impurityelement imparting one conductivity type is added can be concurrentlyformed; thus, the mass productivity can be enhanced. Further, also whenone reaction chamber is being subjected to maintenance or cleaning, thefilms can be formed in other reaction chambers, and the films can beformed efficiently. In addition, an interface between the films can beformed without being contaminated by atmospheric components or impurityelements contained in the atmosphere; thus, variation in characteristicsof the thin film transistors can be reduced.

With use of the microwave plasma CVD apparatus having such a structure,films of similar kinds or one kind can be formed in each reactionchamber, and the films can be successively formed without being exposedto the atmosphere. Thus, an interface between the films can be formedwithout being contaminated by a residue of another film which hasalready been formed or impurity elements contained in the atmosphere.

Further, a microwave generator and a high frequency wave generator maybe provided; thus, the gate insulating film, the microcrystallinesemiconductor film, the channel protective layer, and the semiconductorfilm to which an impurity element imparting one conductivity type isadded may be formed by a microwave plasma CVD method, and the bufferlayer may be formed by a high frequency plasma CVD method.

Although the microwave plasma CVD apparatus in FIGS. 13A and 13B isprovided with the loading chamber and the unloading chamber separately,a loading chamber and an unloading chamber may be combined and aloading/unloading chamber may be provided. In addition, the microwaveplasma CVD apparatus may be provided with a spare chamber. Bypre-heating of the substrate in the spare chamber, it is possible toshorten heating time before formation of the film in each reactionchamber, so that the throughput can be improved. In the film-formationtreatment, a gas supplied from a gas supply portion may be selected inaccordance with its purpose.

This embodiment mode can be combined with the structure disclosed inother embodiment modes, as appropriate.

Embodiment Mode 5

Next, a manufacturing process of a light-emitting device will bedescribed with reference to FIGS. 10A and 10B and FIGS. 11A to 11C. Alight-emitting device, in which a light-emitting element utilizingelectroluminescence is used, is described here. Light-emitting elementsutilizing electroluminescence are classified according to whether alight-emitting material is an organic compound or an inorganic compound.In general, the former is referred to as organic EL elements and thelatter as inorganic EL elements. Thin film transistors 85 and 86 usedfor the light-emitting device can be formed in a similar manner to thethin film transistor 74 or 274 shown in Embodiment Mode 1 or 2 and havehigh electric characteristics and high reliability.

In an organic EL element, by application of a voltage to alight-emitting element, electrons and holes are separately injected froma pair of electrodes into a layer containing a light-emitting organiccompound, and current flows. Then, recombination of these carriers (theelectrons and holes) causes the light-emitting organic compound to be inan excited state and to emit light when it returns from the excitedstate to a ground state. Due to such a mechanism, such a light-emittingelement is referred to as a current-excitation light-emitting element.

Inorganic EL elements are classified into a dispersion type inorganic ELelement and a thin-film type inorganic EL element, depending on theirelement structures. A dispersion type inorganic EL element has alight-emitting layer where particles of a light-emitting material aredispersed in a binder, and its light emission mechanism isdonor-acceptor recombination type light emission that utilizes a donorlevel and an acceptor level. A thin-film type inorganic EL element has astructure where a light-emitting layer is sandwiched between dielectriclayers, which are further sandwiched between electrodes, and its lightemission mechanism is localized type light emission that utilizesinner-shell electron transition of metal ions. Note that description ismade here using an organic EL element as a light-emitting element. Inaddition, description is made using the channel stop type thin filmtransistor shown in FIG. 1 as a thin film transistor which controlsdriving of a light-emitting element.

Through the process of FIG. 1, FIGS. 2A to 2D, FIGS. 3A to 3C, and FIGS.4A to 4D, the thin film transistors 85 and 86 are formed over asubstrate 100, and an insulating film 87 functioning as a protectivefilm is formed over the thin film transistors 85 and 86 as shown inFIGS. 10A and 10B. Next, a planarization film 111 is formed over theinsulating film 87, and a pixel electrode 112 connected to a source ordrain electrode of the thin film transistor 86 is formed over theplanarization film 111.

It is preferable that the planarization film 111 be formed using anorganic resin such as acrylic, polyimide, or polyamide, or usingsiloxane.

In FIG. 10A, the thin film transistor of a pixel is an n-type thin filmtransistor; thus, it is desired that the pixel electrode 112 be acathode. In contrast, when the thin film transistor is a p-type thinfilm transistor, it is desired that the pixel electrode 112 be an anode.Specifically, as a cathode, a known material with low work function,such as Ca, Al, CaF, MgAg, or AlLi, can be used.

Next, as shown in FIG. 10B, a partition 113 is formed over theplanarization film 111 and an end portion of the pixel electrode 112.The partition 113 has an opening, through which the pixel electrode 112is exposed. The partition 113 is formed using an organic resin film, aninorganic insulating film, or organic polysiloxane. It is particularlypreferable that the partition be formed using a photosensitive materialto have an opening over the pixel electrode so that a sidewall of theopening is formed as a tilted surface with continuous curvature.

Next, a light-emitting layer 114 is formed in contact with the pixelelectrode 112 in the opening of the partition 113. The light-emittinglayer 114 may be formed using either a single layer or a stacked layerof a plurality of layers.

Then, a common electrode 115 serving as an anode is formed to cover thelight-emitting layer 114. The common electrode 115 can be formed of alight-transmitting conductive film using any of the light-transmittingconductive materials listed in Embodiment Mode 1 for the pixel electrode77. As the common electrode 115, a titanium nitride film or a titaniumfilm may be used in addition to the above-mentioned light-transmittingconductive film. In FIG. 10B, ITO is used for the common electrode 115.In the opening of the partition 113, a light-emitting element 117 isformed by overlapping of the pixel electrode 112, the light-emittinglayer 114, and the common electrode 115. After that, it is preferablethat a protective film 116 be formed over the common electrode 115 andthe partition 113 so that oxygen, hydrogen, moisture, carbon dioxide, orthe like does not enter the light-emitting element 117. As theprotective film 116, a silicon nitride film, a silicon nitride oxidefilm, a DLC film, or the like can be formed.

Furthermore, practically, after the steps to the step of FIG. 10B arecompleted, it is preferable that packaging (encapsulation) be performedusing a protective film (a laminated film, an ultraviolet curable resinfilm, or the like), which has high airtightness and causes lessdegasification, in order to prevent further exposure to external air.

Next, structures of light-emitting elements are described with referenceto FIGS. 11A to 11C. Here, the case where a driving TFT is of an n-typeis given as an example, and cross-sectional structures of pixels aredescribed. Driving TFTs 7001, 7011, and 7021 used for light-emittingdevices shown in FIGS. 11A to 11C can be formed in a similar manner tothe thin film transistor 74 or 274 shown in Embodiment Modes 1 to 4 andhave high electric characteristics and high reliability.

In a light-emitting element, it is acceptable as long as at least one ofan anode and a cathode is transparent in order to extract lightemission. There are light-emitting elements having the followingstructures: a top emission structure where a thin film transistor and alight-emitting element are formed over a substrate and light isextracted from a side opposite to the substrate; a bottom emissionstructure where light is extracted from the substrate side; and a dualemission structure where light is extracted from both the substrate sideand the side opposite to the substrate. The pixel structure of thepresent invention can be applied to a light-emitting element with any ofthe emission structures.

A light-emitting element having a top emission structure is describedwith reference to FIG. 11A.

FIG. 11A is a cross-sectional view of a pixel in the case where thedriving TFT 7001 is of an n-type and light is emitted from alight-emitting element 7002 to an anode 7005 side. In FIG. 11A, acathode 7003 of the light-emitting element 7002 is electricallyconnected to the driving TFT 7001, and a light-emitting layer 7004 andthe anode 7005 are stacked in sequence over the cathode 7003. Thecathode 7003 can be formed using various conductive materials as long asthey have a low work function and reflect light. For example, Ca, Al,CaF, MgAg, AlLi, or the like is preferable. The light-emitting layer7004 may be formed using either a single layer or a stacked layer of aplurality of layers. In the case of using a plurality of layers, anelectron injection layer, an electron transport layer, a light-emittinglayer, a hole transport layer, and a hole injection layer are stacked inthis order over the cathode 7003. Note that all of these layers are notnecessarily provided. The anode 7005 is formed using alight-transmitting conductive material that transmits light, and forexample, a light-transmitting conductive film of indium oxide containingtungsten oxide, indium zinc oxide containing tungsten oxide, indiumoxide containing titanium oxide, indium tin oxide containing titaniumoxide, indium tin oxide (hereinafter referred to as ITO), indium zincoxide, indium tin oxide to which silicon oxide has been added, or thelike may be used.

A region where the light-emitting layer 7004 is sandwiched between thecathode 7003 and the anode 7005 corresponds to the light-emittingelement 7002. In the case of the pixel shown in FIG. 11A, light isemitted from the light-emitting element 7002 to the anode 7005 side asindicated by an arrow.

Next, a light-emitting element having a bottom emission structure isdescribed with reference to FIG. 11B. FIG. 11B is a cross-sectional viewof a pixel in the case where the driving TFT 7011 is of an n-type andlight is emitted from a light-emitting element 7012 to a cathode 7013side. In FIG. 11B, the cathode 7013 of the light-emitting element 7012is formed over a light-transmitting conductive film 7017 which iselectrically connected to the driving TFT 7011, and a light-emittinglayer 7014 and an anode 7015 are sequentially stacked over the cathode7013. Note that, in the case where the anode 7015 has alight-transmitting property, a blocking film 7016 for reflecting orblocking light may be formed to cover the anode. As in the case of FIG.11A, the cathode 7013 can be formed using various conductive materialsas long as they have a low work function. Note that the thickness is setsuch that light is transmitted therethrough (preferably, about 5 nm to30 nm). For example, an aluminum film with a thickness of 20 nm can beused as the cathode 7013. As in the case of FIG. 11A, the light-emittinglayer 7014 may be formed using either a single layer or a stacked layerof a plurality of layers. Although the anode 7015 does not need to beable to transmit light, similarly to FIG. 11A, it can be formed using alight-transmitting conductive material. The blocking film 7016 can beformed using, for example, a metal which reflects light, or the like;however, the present invention is not limited to a metal film. Forexample, a resin to which black colorant is added can also be used.

A region where the light-emitting layer 7014 is sandwiched between thecathode 7013 and the anode 7015 corresponds to the light-emittingelement 7012. In the case of the pixel shown in FIG. 11B, light isemitted from the light-emitting element 7012 to the cathode 7013 side asindicated by an arrow.

Next, a light-emitting element having a dual emission structure isdescribed with reference to FIG. 11C. In FIG. 11C, a cathode 7023 of alight-emitting element 7022 is formed over a light-transmittingconductive film 7027 which is electrically connected to the driving TFT7021, and a light-emitting layer 7024 and an anode 7025 are sequentiallystacked over the cathode 7023. As in the case of FIG. 11A, the cathode7023 can be formed using various conductive materials as long as theyhave a low work function. Note that the thickness is set such that lightis transmitted therethrough. For example, an Al film with a thickness of20 nm can be used as the cathode 7023. As in FIG. 11A, thelight-emitting layer 7024 may be formed using either a single layer or astacked layer of a plurality of layers. Similar to FIG. 11A, the anode7025 can be formed using a light-transmitting conductive material whichtransmits light.

A region where the cathode 7023, the light-emitting layer 7024, and theanode 7025 overlap with each other corresponds to the light-emittingelement 7022. In the case of the pixel shown in FIG. 11C, light isemitted from the light-emitting element 7022 to both the anode 7025 sideand the cathode 7023 side as indicated by arrows.

Note that, although an organic EL element is described here as alight-emitting element, an inorganic EL element can also be provided asa light-emitting element.

Note that, in this embodiment mode, the example is described in which athin film transistor (a driving TFT) which controls the driving of alight-emitting element is electrically connected to the light-emittingelement, but a structure may be employed in which a TFT for currentcontrol is connected between the driving TFT and the light-emittingelement.

Note that the light-emitting device described in this embodiment mode isnot limited to the structures shown in FIGS. 11A to 11C and can bemodified in various ways based on the technical idea of the presentinvention.

Through the above-described process, a light-emitting device can bemanufactured. Since a thin film transistor with high electriccharacteristics and high reliability is used in the light-emittingdevice of this embodiment mode, the light-emitting device has highcontrast and high visibility.

This embodiment mode can be implemented in combination with thestructures of other embodiment modes as appropriate.

Embodiment Mode 6

Next, a structure of a light-emitting display panel (also referred to asa light-emitting panel), which is one mode of a light-emitting device ofthe present invention, is described below.

FIG. 9A shows a mode of a light-emitting display panel in which a signalline driver circuit 6013 which is separately formed is connected to apixel portion 6012 formed over a substrate 6011. The pixel portion 6012and a scanning line driver circuit 6014 are each formed using a thinfilm transistor in which a microcrystalline semiconductor film is used.When the signal line driver circuit is formed using a transistor inwhich higher field-effect mobility can be obtained compared to the thinfilm transistor in which the microcrystalline semiconductor film isused, an operation of the signal line driver circuit which demandshigher driving frequency than that of the scanning line driver circuitcan be stabilized. Note that the signal line driver circuit 6013 may beformed using a thin film transistor using a single crystallinesemiconductor, a thin film transistor using a polycrystallinesemiconductor, or a thin film transistor using SOI. The pixel portion6012, the signal line driver circuit 6013, and the scanning line drivercircuit 6014 are each supplied with a potential of a power supply, avariety of signals, and the like via an FPC 6015.

Note that both the signal line driver circuit and the scanning linedriver circuit may be formed over the same substrate as that of thepixel portion.

When a driver circuit is separately formed, a substrate over which thedriver circuit is formed is not necessarily attached to a substrate overwhich a pixel portion is formed, and may be attached to an FPC, forexample. FIG. 9B shows a mode of a light-emitting panel in which asignal line driver circuit 6023 which is separately formed is connectedto a pixel portion 6022 and a scanning line driver circuit 6024 formedover a substrate 6021. The pixel portion 6022 and the scanning linedriver circuit 6024 are each formed using a thin film transistor inwhich a microcrystalline semiconductor film is used. The signal linedriver circuit 6023 is connected to the pixel portion 6022 via an FPC6025. The pixel portion 6022, the signal line driver circuit 6023, andthe scanning line driver circuit 6024 are each supplied with a potentialof a power supply, a variety of signals, and the like via the FPC 6025.

Alternatively, only part of a signal line driver circuit or part of ascanning line driver circuit may be formed over the same substrate as apixel portion by using a thin film transistor in which amicrocrystalline semiconductor film is used, and the other part of thedriver circuit may be separately formed and electrically connected tothe pixel portion. FIG. 9C shows a mode of a light-emitting panel inwhich an analog switch 6033 a included in a signal line driver circuitis formed over a substrate 6031, which is the same substrate as a pixelportion 6032 and a scanning line driver circuit 6034, and a shiftregister 6033 b included in the signal line driver circuit is separatelyformed over a different substrate and attached to the substrate 6031.The pixel portion 6032 and the scanning line driver circuit 6034 areeach formed using a thin film transistor in which a microcrystallinesemiconductor film is used. The shift register 6033 b included in thesignal line driver circuit is connected to the pixel portion 6032 via anFPC 6035. The pixel portion 6032, the signal line driver circuit, andthe scanning line driver circuit 6034 are each supplied with a potentialof a power supply, a variety of signals, and the like via the FPC 6035.

As shown in FIGS. 9A to 9C, in a light-emitting device of the presentinvention, all or part of the driver circuit can be formed over the samesubstrate as that of the pixel portion, using the thin film transistorin which the microcrystalline semiconductor film is used.

Note that a connection method of a substrate which is separately formedis not particularly limited, and a known method such a COG method, awire bonding method, or a TAB method can be used. Further, a connectionposition is not limited to the positions shown in FIGS. 9A to 9C as longas electrical connection is possible. Moreover, a controller, a CPU, amemory, or the like may be formed separately and connected.

Note that a signal line driver circuit used in the present invention isnot limited to a structure including only a shift register and an analogswitch. In addition to the shift register and the analog switch, anothercircuit such as a buffer, a level shifter, or a source follower may beincluded. Moreover, the shift register and the analog switch are notnecessarily provided. For example, a different circuit such as a decodercircuit by which a signal line can be selected may be used instead ofthe shift register, or a latch or the like may be used instead of theanalog switch.

Next, the appearance and a cross section of a light-emitting displaypanel which is one mode of the light-emitting device of the presentinvention are described with reference to FIGS. 12A and 12B. FIG. 12A isa top plan view of a panel. In the panel, a thin film transistor inwhich a microcrystalline semiconductor film is used and a light-emittingelement which are formed over a first substrate are sealed between thefirst substrate and a second substrate by a sealing material. FIG. 12Bis a cross-sectional view of a cross section taken along a line E-F inFIG. 12A.

The sealing material 4505 is provided so as to surround a pixel portion4502 and a scanning line driver circuit 4504 which are provided over afirst substrate 4501. A second substrate 4506 is provided over the pixelportion 4502 and the scanning line driver circuit 4504. Accordingly, thepixel portion 4502 and the scanning line driver circuit 4504 are sealedtogether with a filler 4507 by the first substrate 4501, the sealingmaterial 4505, and the second substrate 4506. Further, a signal linedriver circuit 4503 formed using a polycrystalline semiconductor filmover a different substrate is mounted on a region over the firstsubstrate 4501, which is different from the region surrounded by thesealing material 4505. Note that in this embodiment mode, an example isdescribed in which the signal line driver circuit including a thin filmtransistor using a polycrystalline semiconductor film is attached to thefirst substrate 4501; however, a signal line driver circuit may beformed using a transistor using a single crystalline semiconductor andattached to a substrate. FIGS. 12A and 12B illustrate a thin filmtransistor 4509 formed using a polycrystalline semiconductor film, whichis included in the signal line driver circuit 4503.

Each of the pixel portion 4502 and the scanning line driver circuit 4504which are provided over the first substrate 4501 includes a plurality ofthin film transistors. FIG. 12B illustrates a thin film transistor 4510included in the pixel portion 4502. In this embodiment mode, the thinfilm transistor 4510 is illustrated as a driving TFT but may also be acurrent control TFT or an erasing TFT. The thin film transistor 4510corresponds to a thin film transistor which uses a microcrystallinesemiconductor film and can be formed through a similar manufacturingprocess to that of Embodiment Modes 1 to 3.

A light-emitting element 4511 includes a pixel electrode electricallyconnected to a source or drain electrode of the thin film transistor4510 through a wiring 4517. In this embodiment mode, a common electrodeof the light-emitting element 4511 and a light-emitting conductive film4512 are electrically connected to each other. The structure of thelight-emitting element 4511 is not limited to the structure described inthis embodiment mode. The structure of the light-emitting element 4511can be changed as appropriate in accordance with a direction of lighttaken from the light-emitting element 4511, polarity of the thin filmtransistor 4510, or the like.

A variety of signals and potential which are applied to the signal linedriver circuit 4503 that is formed separately, the scanning line drivercircuit 4504, or the pixel portion 4502 are supplied from an FPC 4518through a wiring 4514 and a wiring 4515, although not illustrated in thecross-sectional view of FIG. 12B.

In this embodiment mode, a connecting terminal 4516 is formed of thesame conductive film as that of the pixel electrode included in thelight-emitting element 4511. In addition, the wirings 4514 and 4515 areformed of the same conductive film as that of the wiring 4517.

The connecting terminal 4516 is electrically connected to a terminalincluded in the FPC 4518 through an anisotropic conductive film 4519.

Note that the second substrate in a direction to extract light from thelight-emitting element 4511 needs to be transparent. In that case, alight-transmitting material such as a glass plate, a plastic plate, apolyester film, or an acrylic film is used.

As the filler 4507, an ultraviolet curable resin or a thermosettingresin can be used, in addition to an inert gas such as nitrogen orargon. For example, PVC (polyvinyl chloride), acrylic, polyimide, anepoxy resin, a silicone resin, PVB (polyvinyl butyral), or EVA (ethylenevinyl acetate) can be used. In this embodiment mode, nitrogen is used asthe filler.

If necessary, a polarizing plate, a circularly polarizing plate(including an elliptically polarizing plate), a retardation plate (aquarter-wave plate or a half-wave plate), or an optical film such as acolor filter may be provided as appropriate over a light-emittingsurface of the light-emitting element. Further, a polarizing plate or acircularly polarizing plate may be provided with an anti-reflectionfilm. For example, anti-glare treatment may be carried out by whichreflected light can be diffused by projections and depressions on asurface so as to reduce reflection.

FIGS. 12A and 12B show an example in which the signal line drivercircuit 4503 is formed separately and mounted on the first substrate4501, but this embodiment mode is not limited to this structure. Thescanning line driver circuit may be formed separately and then mounted,or only part of the signal line driver circuit or part of the scanningline driver circuit may be formed separately and then mounted.

This embodiment mode can be implemented in combination with thestructures of other embodiment modes as appropriate.

Embodiment Mode 7

Light-emitting devices that are obtained according to the presentinvention and the like can be used for light-emitting display modules(e.g., active matrix EL modules). That is to say, the present inventioncan be carried out in all electronic devices in which these modules areincorporated into display portions.

As such electronic devices, cameras such as video cameras and digitalcameras; displays that can be mounted on a person's head (goggle-typedisplays); car navigation systems; projectors; car stereos; personalcomputers; portable information terminals (e.g., mobile computers,mobile phones, and electronic books); and the like can be given.Examples of these devices are illustrated in FIGS. 7A to 7D.

FIG. 7A shows a television device. A television device can be completedby incorporation of a light-emitting display module into a chassis asshown in FIG. 7A. A light-emitting display panel including components upto an FPC is also referred to as a light-emitting display module. A mainscreen 2003 is formed with a light-emitting display module. In addition,a speaker unit 2009, operation switches, and the like are provided asaccessory equipment. In this manner, a television device can becompleted.

As shown in FIG. 7A, a light-emitting display panel 2002 includinglight-emitting elements is incorporated into a chassis 2001. In additionto reception of general television broadcast by a receiver 2005,communication of information in one direction (from a transmitter to areceiver) or in two directions (between a transmitter and a receiver orbetween receivers) can be performed by connection to a wired or wirelesscommunication network through a modem 2004. The television device can beoperated using switches that are incorporated into the chassis or by aremote control device 2006 that is provided separately, and a displayportion 2007 that displays information which is to be output may beprovided for the remote control device.

Further, in the television device, a sub-screen 2008 may be formed usinga second light-emitting display panel and used to display channelnumber, volume, and the like, in addition to the main screen 2003.

FIG. 8 is a block diagram showing the main structure of the televisiondevice. A pixel portion 901 is formed in the light-emitting displaypanel. A signal line driver circuit 902 and a scanning line drivercircuit 903 may be mounted on the light-emitting display panel by a COGmethod.

As another external circuit, a video signal amplifier circuit 905 whichamplifies a video signal among signals received by a tuner 904, a videosignal processing circuit 906 which converts the signals output from thevideo signal amplifier circuit 905 into chrominance signalscorresponding to red, green, and blue, a control circuit 907 whichconverts the video signal into an input specification of the driver IC,and the like are provided on an input side of the video signal. Thecontrol circuit 907 outputs signals to a scanning line side and a signalline side. In the case of digital driving, a signal dividing circuit 908may be provided on the signal line side and an input digital signal maybe split into m pieces to be supplied.

Among signals received by the tuner 904, an audio signal is transmittedto an audio signal amplifier circuit 909, and the output thereof issupplied to a speaker 913 through an audio signal processing circuit910. A control circuit 911 receives control information of a receivingstation (reception frequency) or sound volume from an input portion 912and transmits signals to the tuner 904 and the audio signal processingcircuit 910.

The present invention is not limited to the television device and isalso applicable to various usages such as display mediums having a largearea, for example, a monitor of a personal computer, an informationdisplay board at a railway station, an airport, or the like, or anadvertisement display board on the street.

FIG. 7B illustrates one mode of a mobile phone 2301. The mobile phone2301 includes a display portion 2302, operation switches 2303, and thelike. The light-emitting device described in the preceding embodimentmodes is applied to the display portion 2302, so that reliability andmass productivity of the mobile phone can be improved.

A portable computer illustrated in FIG. 7C includes a main body 2401, adisplay portion 2402, and the like. The light-emitting device describedin the preceding embodiment modes is applied to the display portion2402, so that reliability and mass productivity of the portable computercan be improved.

FIG. 7D shows a desk lamp including a lighting portion 2501, a shade2502, an adjustable arm 2503, a support 2504, a base 2505, and a powersupply switch 2506. The desk lamp can be manufactured by applying thelight-emitting device of the present invention to the lighting portion2501. Note that the lighting equipment includes a ceiling light, a walllight, and the like. The light-emitting device described in thepreceding embodiment modes is applied to the lighting portion 2501, sothat reliability and mass productivity of the desk lamp can be improved.

This application is based on Japanese Patent Application Serial No.2007-190217 filed with Japan Patent Office on Jul. 20, 2007, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A display device comprising: a gate electrodeover a substrate; a first insulating film over the gate electrode; amicrocrystalline semiconductor film including a channel formationregion, the microcrystalline semiconductor film over the gate electrodewith the first insulating film interposed therebetween; an amorphoussemiconductor film over and being in contact with the microcrystallinesemiconductor film; a pair of semiconductor films comprising an impurityelement imparting one conductivity type, the pair of semiconductor filmsover the microcrystalline semiconductor film; and a second insulatingfilm over the microcrystalline semiconductor film, the amorphoussemiconductor film, and the pair of semiconductor films, wherein endportions of the microcrystalline semiconductor film are positioned moreinwardly than end portions of the gate electrode, and wherein theamorphous semiconductor film contains nitrogen, carbon, and oxygen at atotal concentration of greater than or equal to 1×10²⁰ atoms/cm³ andless than or equal to 15×10²⁰ atoms/cm³.
 2. The display device accordingto claim 1, wherein the gate electrode has an upper surface and atapered-shape side surface, wherein the microcrystalline semiconductorfilm is over the upper surface of the gate electrode, and wherein themicrocrystalline semiconductor film is not overlapped with thetapered-shape side surface.
 3. An electronic device having the displaydevice according to claim 1, wherein the electronic device is oneselected from the group consisting of a camera, a goggle-type display, acar navigation system, a projector, a car stereo, a personal computer, aportable information terminal, a television device, and a lamp.
 4. Adisplay device comprising: a gate electrode over a substrate; aninsulating film over the gate electrode; a microcrystallinesemiconductor film including a channel formation region, themicrocrystalline semiconductor film over the gate electrode with theinsulating film interposed therebetween; an amorphous semiconductor filmover and being in contact with the microcrystalline semiconductor film;a channel protective layer over the amorphous semiconductor film, thechannel protective layer overlapping the channel formation region; and apair of semiconductor films comprising an impurity element imparting oneconductivity type, the pair of semiconductor films over the channelprotective layer, wherein end portions of the microcrystallinesemiconductor film are positioned more inwardly than end portions of thegate electrode, wherein end portions of the channel protective layer arepositioned more inwardly than the end portions of the microcrystallinesemiconductor film, and wherein the amorphous semiconductor filmcontains nitrogen, carbon, and oxygen at a total concentration ofgreater than or equal to 1×10²⁰ atoms/cm³ and less than or equal to15×10²⁰ atoms/cm³.
 5. The display device according to claim 4, whereinthe gate electrode has an upper surface and a tapered-shape sidesurface, wherein the microcrystalline semiconductor film is over theupper surface of the gate electrode, and wherein the microcrystallinesemiconductor film is not overlapped with the tapered-shape sidesurface.
 6. The display device according to claim 4, wherein the channelprotective layer comprises a material selected from the group consistingof silicon oxide, silicon nitride, silicon oxynitride, and siliconnitride oxide.
 7. The display device according to claim 4, wherein thechannel protective layer comprises a material selected from the groupconsisting of a photosensitive organic material and a non-photosensitiveorganic material.
 8. An electronic device having the display deviceaccording to claim 4, wherein the electronic device is one selected fromthe group consisting of a camera, a goggle-type display, a carnavigation system, a projector, a car stereo, a personal computer, aportable information terminal, a television device, and a lamp.
 9. Adisplay device comprising: a gate electrode over a substrate; aninsulating film over the gate electrode; a microcrystallinesemiconductor film including a channel formation region, themicrocrystalline semiconductor film over the gate electrode with theinsulating film interposed therebetween; an amorphous semiconductor filmover and being in contact with the microcrystalline semiconductor film;a channel protective layer over and being in contact with the amorphoussemiconductor film, the channel protective layer overlapping the channelformation region; and a pair of semiconductor films comprising animpurity element imparting one conductivity type, the pair ofsemiconductor films over the amorphous semiconductor film and thechannel protective layer, wherein end portions of the microcrystallinesemiconductor film are positioned more inwardly than end portions of thegate electrode, and wherein the amorphous semiconductor film containsnitrogen, carbon, and oxygen at a total concentration of greater than orequal to 1×10²⁰ atoms/cm³ and less than or equal to 15×10²⁰ atoms/cm³.10. The display device according to claim 9, wherein the gate electrodehas an upper surface and a tapered-shape side surface, wherein themicrocrystalline semiconductor film is over the upper surface of thegate electrode, and wherein the microcrystalline semiconductor film isnot overlapped with the tapered-shape side surface.
 11. The displaydevice according to claim 9, wherein the channel protective layercomprises a material selected from the group consisting of siliconoxide, silicon nitride, silicon oxynitride, and silicon nitride oxide.12. The display device according to claim 9, wherein the channelprotective layer comprises a material selected from the group consistingof a photosensitive organic material and a non-photosensitive organicmaterial.
 13. An electronic device having the display device accordingto claim 9, wherein the electronic device is one selected from the groupconsisting of a camera, a goggle-type display, a car navigation system,a projector, a car stereo, a personal computer, a portable informationterminal, a television device, and a lamp.
 14. A display devicecomprising: a gate electrode; a first insulating film over the gateelectrode; a microcrystalline semiconductor film including a channelformation region over the first insulating film; an amorphoussemiconductor film over and being in contact with the microcrystallinesemiconductor film; a channel protective layer over and being in contactwith the amorphous semiconductor film, the channel protective layeroverlapping the channel formation region; a source region and a drainregion formed over the channel protective layer and the amorphoussemiconductor film; a source electrode and a drain electrode formed overthe source region and the drain region; a second insulating film formedover the channel protective layer, the source electrode, and the drainelectrode; a pixel electrode electrically connected to one of the sourceelectrode and the drain electrode; a partition formed over the pixelelectrode; and a light emitting layer formed over the pixel electrodeand the partition, wherein end portions of the microcrystallinesemiconductor film are positioned more inwardly than end portions of thegate electrode, and wherein the amorphous semiconductor film containsnitrogen, carbon, and oxygen at a total concentration of greater than orequal to 1×10²⁰ atoms/cm³ and less than or equal to 15×10²⁰ atoms/cm³.15. The display device according to claim 14, further comprising acommon electrode over the light emitting layer.
 16. The display deviceaccording to claim 14, wherein an area of the source region and thedrain region is larger than an area of the source electrode and thedrain electrode.
 17. The display device according to claim 14, whereinthe gate electrode has an upper surface and a tapered-shape sidesurface, wherein the microcrystalline semiconductor film is over theupper surface of the gate electrode, and wherein the microcrystallinesemiconductor film is not overlapped with the tapered-shape sidesurface.
 18. The display device according to claim 14, wherein thechannel protective layer comprises a material selected from the groupconsisting of silicon oxide, silicon nitride, silicon oxynitride, andsilicon nitride oxide.
 19. The display device according to claim 14,wherein the channel protective layer comprises a material selected fromthe group consisting of a photosensitive organic material and anon-photosensitive organic material.
 20. An electronic device having thedisplay device according to claim 14, wherein the electronic device isone selected from the group consisting of a camera, a goggle-typedisplay, a car navigation system, a projector, a car stereo, a personalcomputer, a portable information terminal, a television device, and alamp.
 21. The display device according to claim 14, wherein end portionsof the channel protective layer are positioned more inwardly than theend portions of the microcrystalline semiconductor film.
 22. The displaydevice according to claim 14, wherein an area of the source region andthe drain region is larger than an area of the source electrode and thedrain electrode.